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Not sure whether this is the right section. It's a build/hack involving powered speakers which only have USB input, so run the amp module via a digital to analog convertor chip (the relatively common UAC3552A - similar to the ones used in mp3 players for headphones). Feel free to move if I'm in the wrong section.

I wondered whether any of you audio / electronics wizards could help with a speaker hack I'm attempting as a side-project (am waiting for some electronics for another project from China, so I have time!).

I scored some Harman Kardon soundsticks from Freecycle - listed as "power up and make a pop when USB cable is plugged in, but don't get recognised by computer"

I thought, sweet - the amplifier and power supply must be fine, digital USB interface / cable is goosed, but who needs that, I have lots of analog sound I can feed it. And so began the thoughts of simply wiring in an aux input to the analog side of the board (the USB digital signal is converted to an analog signal by a UAC3552A IC).

I added an AUX once before to an old stereo (way before the internet) and it worked fine, however things seem more complicated. The things that confuse me are:

(1) there seems to be a need to "bypass" some kind of mute function on the amp.(2) the Japanse dude seems to add resistors into his analog feed, why is this done? I've never seen that before.

for (1) and (2) see picture below:

(3) I've located the datasheet for the UAC3552A and have seen it incorporates an AUX in function - diagrams below:

I'd like some help understanding the diagrams for AUXEN and AUXL / AUXR pins, I assume the triangle is the chip itself

In summary, I think hooking up the AUX in to a switch will do the trick, hopefully it enable AUX in, "un-mute" the amplifier and allow an analog signal through.

I propose to:

(a) Cut the pin / trace on Pin 33 (AUXEN) and them solder a jumper wire (possibly with a switch) directly between pin 33 (AUXEN) to pin 28 (VSS); and

(b) hook up an AUX lead with the L and R going to pins 36 and 37 (AUXL and AUXR), and the ground to a ground point somewhere (perhaps the one Japanese dude hilights in his diagram). Will I need 10k resistors in here somewhere to avoid damaging the chip?

As I got the speakers for free, I don't mind having a poke about, but would like to minimise chances of blowing up some salvageable speakers!

I don't know if you already modified your USB soundsticks but if you are still looking for a solution, I have done the conversion USB to jack 3.5mm on my Harman Kardon Soundsticks I this week.

On my point of view, the easiest way is to remove the chip UAC 3552A and connect your analog input instead of OUTL and OUTR UAC3552A chip. You can use the original USB cable to do that. If you need more détails, you can have a look here : https://www.logre.eu/wiki/Soundsticks_USB_vers_jack and don't hesitate to ask me.

greg, thank you for this article I have been looking for an answer to this problem for a few days now. 1 quick questionCan I just use the two pins AUXL, AUXR by just selecting AUXEN with a small switch one side VSS the other to VDD and the common to AUXEN pin?and then put some shielded wires to AUXL, AUXR to a 3mm jack? and then a 3mm to 3mm jumper between the speakers and the computer.Fins.

I have the same problem as well. But after reading your article 'https://www.logre.eu/wiki/Soundsticks_USB_vers_jack' I think I have a different take on it.I still want the USB connector (option) but I also want a 'line input' as well.I propose the following:I wish to wire AUXEN to a DPDT switch (common) and then Vss and Vdd to the Switch NO & NC. Install a 3mm Stereo Jack, and connect L & R to AUXL and AUXR using shielded wire. you will most likely need a voltage divider as well. Thanks, Cris H.

I have made two posts, one on the EEVblog forums and one on the AskElectronics subreddit asking for help to carry out this mod. I am posting the links here in the hopes that someone can help me out with the circuit.

Looking at the sch of the UAC3552A. The aux-en is pulled to ground threw the 11K <R18> when not used. Enabling the aux inputs is done by taking the aux-en to 5v+ threw a 150K (R16). Yes you can use a switch to select between aux/analogue and digital providing those resistors are present on the board. Connect the sw directly to board.

If the res are not on the board however, (sometimes the case if this feature is not provided from the manufacturer), and, you intend on switching between sources,,Then I would suggest adding them to your switch. It will help bleed off any high voltage spikes that may happen. (walking across the carpet in winter and then touching the sw)(SNAP). Aux-en wired to 150K res going to 5v+, and, one leg of the switch. The other switch leg is taken to GND threw a 11K res or the more common 10K value. Don't forget the series capacitors (470n/0.47uF at 10 to 16 volts will be enough) and the two 11K resistors at the RCA / analogue input jacks. You can use 10K values here to. It will NOT sound good if you forget the caps! If they are soldered on the board already then just connect the wire + jacks and forget about the buys.

P.S. I would try the USB anyways as some comp's have a hard time recognizing some USB sources and assigning a comm port number to them. Other comp's have no problem assigning a comp port number to anything plugged into it. Go figure!

Inputs are held to a reference voltage of 2.25v which is close to 1/2 supply voltage. This is a good place allowing +/- 2.25v input signal. Perfect for CD's, Comp Out, Tuners, Tape Decks, etc.

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